Structure and Dynamics of Dinuclear Zirconium(IV) Complexes

We have determined by X-ray crystallography the structures of three dinuclear zirconium(IV) complexes containing
the heptadentate ligand dhpta (where H5dhpta = 1,3-diamino-2-propanol-N,N,N‘,N‘-tetraacetic acid, 1) and different
countercations:  K2[Zr2(dhpta)2]·5H2O (2·5H2O), Na2[Zr2(dhpta)2]·7H2O·C2H5OH (3·7H2O·C2H5OH), and Cs2[Zr2(dhpta)2]·H5O2·Cl·4H2O (4·H5O2·Cl·4H2O). In the K(I) complex 2, crystallized from water, the two Zr(IV) ions are
3.5973(4) Å apart and bridged via two alkoxo groups (average Zr−O 2.165 Å). Each Zr(IV) is eight-coordinate and
also bound to two N atoms (average Zr−N 2.448 Å), and four carboxylate O atoms (average Zr−O 2.148 Å). The
two dhpta ligands in the dinuclear unit have different conformations. One face of the complex contains an array of
14 oxygen atoms and interacts strongly with the two K(I) ions, one of which is 6-coordinate, the other 8-coordinate,
which are 3.922(4) Å apart and bridged by a carboxylate O and by two water molecules. The structures of the
dinuclear anion [Zr2(dhpta)2]2- in the Na(I) complex 3 and in the Cs(I) complex 4 are essentially identical to that
found in complex 2, although the alkali metal ions coordinate differently to the oxygen-rich face. All Zr(IV) ions
have a distorted triangulated dodecahedral geometry. Although the crystal structure of complex 2 does not indicate
the presence of acidic protons, in 4 an [H5O2]+ unit is strongly H-bonded to an oxygen atom of a coordinated
carboxylate group. 1D and 2D 1H and 13C NMR spectroscopic and potentiometric studies reveal two deprotonations
with pKa values of 9.0 and 10.0. At low pH, two carboxylate groups appear to undergo protonation accompanied
by chelate ring-opening, and the complex exhibits dynamic fluxional behavior in which the two magnetically
nonequivalent dhpta ligands exchange at a rate of 11 s-1 at pH 3.30, 298 K, as determined from 2D EXSY NMR
studies. Ligand interchange is not observed at high pH (>11). The same crystals of complex 2 were obtained from
solutions at pH 3 or 12. The dynamic configurational change is therefore mediated by the aqueous solvent.